1. Field of the Invention
The invention relates to methods and apparatus for producing thin films on substrates by vapor deposition (e.g., sputtering). The invention pertains to improving the accuracy of the deposited film thickness across the substrate (e.g., by improving the uniformity of the film thickness across the substrate where it is desired to deposit a film having uniform thickness). The invention presents the particular advantage of avoiding the use of apertures or masks to tailor the deposition profile.
2. Discussion of the Related Art
Thin film coatings are typically produced by various vapor deposition methods (such as sputtering, chemical vapor deposition (CVD), electron beam evaporation, thermal evaporation, and ion beam plating) in which the substrate to be coated is passed through a vapor of the coating material and accumulates a thin film through condensation of the vapor. For many applications, such as optical films for EUV (extreme ultraviolet) lithography, it is desirable that the coating be very uniform in thickness (e.g., with no more than 0.1% variation in thickness across the coated substrate). Multilayer coatings for EUV optics are commonly applied using DC magnetron sputtering.
FIG. 1 is a side cross-sectional view of a DC magnetron sputtering system, and FIG. 2 is a cross-sectional view of the FIG. 1 system taken along line 2xe2x80x942 of FIG. 1. The system of FIGS. 1 and 2, described in U.S. Pat. No. 6,010,600, issued Jan. 4, 2000, to Vernon and Ceglio (assigned to the assignee of the present application), includes housing 10 (which has a cylindrical sidewall) and two rectangular sources (of sputtered atoms) located 180 degrees apart (relative to the system""s vertical central axis through the center of shaft 6) at opposite sides of housing 10. One source is surrounded by chimney 2; the other is surrounded by chimney 2A. Chimneys 2 and 2A limit the deposition zone for each source (in which sputtered atoms can be deposited on substrate 11 or 12) to the area directly above the target (3 or 3A) of each source. The two substrates (11 and 12) are held face down on rotatable platter 5 above the sources, at locations 90 degrees apart with respect to the axis of shaft 6.
Multilayers (alternating layers of two different materials) can be deposited on each of substrates 11 and 12 by sweeping the substrates across the sources (by controlled rotation of shaft 6 and hence platter 5 relative to the stationary housing 10 and the stationary sources).
More specifically, the system of FIGS. 1 and 2 includes a first source comprising magnetron 1, chimney 2, and target 3 positioned within chimney 2 in the electric and magnetic fields produced by element 1 (such that ions present within chimney 2, e.g., ions created within chimney 2, will accelerate toward and be incident on target 3). In response to collisions of the ions (which can be argon ions) with target 3, a vapor of sputtered atoms 4 is produced in the volume surrounded by chimney 2. Some of atoms 4 will be deposited on the downward-facing surface of substrate 11 (or 12), when the substrate (11 or 12) is exposed to sputtered atoms 4 in chimney 2.
Similarly, the system also includes a second source comprising magnetron 1A, chimney 2A, and target 3A positioned within chimney 2A in the electric and magnetic fields produced by element 1A (such that ions created within chimney 2A will accelerate toward and be incident on target 3A). In response to collisions of the ions with target 3A, a vapor of sputtered atoms 4A is produced in the volume surrounded by chimney 2A. Some of atoms 4A will be deposited on the downward-facing surface of substrate 11 (or 12), when the substrate (11 or 12) is exposed to sputtered atoms 4A in chimney 2A.
Each of two substrate holders 9 (only one of which is visible in FIG. 1) is fixedly mounted to the lower end of one of shafts 8 (only one of which is visible in FIG. 1) so as to fit in an orifice extending through platter 5. Substrate 11 is mounted on substrate holder 9, with a downward facing surface to be coated. Shaft 8 is rotatably connected to spinner 7, so that spinner 7 can cause shaft 8, holder 9, and substrate 11 to rotate as a unit relative to platter 5, whether or not platter 5 is itself rotating relative to housing 10. Similarly, substrate 12 (shown in FIG. 2 only) is mounted on a substrate holder 9 (not visible in FIGS. 1 and 2) in turn fixedly mounted to a shaft 8, and the shaft is rotatably connected to a spinner (identical to spinner 7). During operation, elements 1, 2, 3, 1A, 2A, and 3A remain stationary within housing 10, while platter 5 rotates to sweep substrates 11 and 12 sequentially across chimneys 2 and 2A (typically while substrates 11 and 12 are rotated about their centers by the spinners relative to platter 5).
To deposit typical multilayer coatings on the substrates, atoms 4 (in chimney 2) are different (i.e., have a different atomic weight) than atoms 4A in chimney 2A. In some implementations, atoms 4 are Molybdenum atoms and atoms 4A are silicon (or beryllium) atoms (and magnetrons 1 and 1A produce a plasma of ultrapure Argon ions at a pressure of about 1.00 mTorr, with source powers of 360 W and 170 W, respectively, for magnetrons 1A and 1). Platter 5 is rotated within housing 10 (at a first rotational speed) while each substrate spins (at a speed much greater than the first rotational speed) relative to platter 5. During each revolution of platter 5 relative to housing 10, each of substrates 11 and 12 sweeps sequentially across chimney 2 and chimney 2A, so that one layer of atoms 4 and then one layer of atoms 4A condenses on each substrate. The thickness of each layer is determined by the time that the substrate is exposed to the vapor (4 or 4A), which is in turn determined by the substrate transit velocity. The arrangement of substrates 11 and 12 and chimneys 2 and 2A is such that no more than one substrate is over one source at any time. Therefore, the two substrates can be independently coated with identical or completely different multilayer structures.
By rapidly spinning substrate 11 (or 12) about its own axis of symmetry relative to platter 5, good azimuthal uniformity of the condensed coating can be achieved. However, radial non-uniformities in coating thickness typically result.
Spatially graded thin film coatings are typically obtained with carefully shaped masks, or apertures, inserted between the sources and substrates. A different mask is required for each source-substrate combination. The masking operation requires iteration of the shape of the mask, and can be impractical for cases where perfect thickness distribution control is required at the location that coincides with the axis of rotation. At best, this is a tedious and inefficient process that is inappropriate for a robust manufacturing technique. The use of masks or baffles also lacks flexibility when several substrates must be coated in the same deposition run using several deposition sources. If differently customized masks are attached in close proximity to the substrates to intercept part of the incident deposition flux, custom coating gradients can be obtained for each substrate but one cannot independently control the flux emitted by each source. On the other hand, if differently customized masks are installed over the sources to shape the outgoing deposition flux, customized coating gradients can be obtained from each source but one cannot independently control the coating gradients deposited on several different substrates. Finally, since the source distribution flux can change over time as the source or target is consumed, the mask shape may require modification. This would require another mask fabrication and perhaps venting the chamber to atmosphere (undesirable) to install the new mask. As will be apparent from the description below, the present invention avoids use of baffles or masks, allows independent deposition of two different coating distributions on a substrate as the substrate sweeps sequentially across two sources during a single platter rotation, and also allows independent deposition of different coating distributions on several substrates coated in the same deposition run, each of which sweeps sequentially across a source.
U.S. Pat. No. 6,010,600, issued Jan. 4, 2000, to Vernon and Ceglio and U.S. patent application Ser. No. 09/454,673 by Walton, Montcalm and Folta disclose a technique for improving thickness distribution control during vapor deposition and circumvents the noted limitations of conventional masking methods. Instead of masking areas of the source flux, an optimal (or nearly optimal) substrate sweep velocity recipe is determined and the substrate is swept through the deposition zone with a time-varying sweep velocity specified by such recipe. An aspect of the technique is a computer-implemented method for calculating the optimal (or nearly optimal) substrate sweep velocity recipe for obtaining the desired thickness distribution profile, using the measured flux profile from each source as input. The technique disclosed in the parent application is specific to systems in which a substrate moves with time-varying sweep velocity across one or more stationary sources (or in which each source moves with time-varying sweep velocity relative to a stationary substrate), each of the sources emitting a fixed deposition flux. The inventors of the present invention, however, have recognized that it can be difficult to engineer and control such systems in which a load of multiple, heavy substrates (or vapor deposition sources) must be precisely accelerated and decelerated to various velocities within very short distances. The requirements for acceleration/ deceleration of heavy substrates can significantly complicate the design of the substrates"" drive mechanism and increase the cost of the vapor deposition tool.
An important aspect of the present invention is another maskless approach for the production of laterally graded or uniform thin film coatings on arbitrarily shaped substrates, in which one modulates the power applied to each source (or otherwise modulates the flux distribution of each source) instead of modulating the sweep velocity of the substrate relative to each source (or of each source relative to the substrate). The present invention has all the advantages of the velocity modulation method described in the parent application (over conventional masking methods), while eliminating the need for special mechanical drive requirements for modulating substrate (or source) sweep velocity.
Until the present invention, it had not been known how to achieve deposited coating thickness uniformity of better than 0.1% across typical substrates, or how to achieve coating thickness having a precisely predetermined graded (nonuniform) profile across typical substrates (including curved substrates such as EUV optics as well as flat substrates), without the need for modulating substrate (or source) sweep velocity.
Prior to the present invention, it had been known to set the power applied to a vapor deposition source to achieve a desired flux distribution (a desired thickness per unit of time of a layer deposited on a substrate held fixed relative to the source). It had also been known to apply a level of power to a vapor deposition source which determines a desired flux distribution of source, which in turn determines a nominal thickness of a layer (e.g., a thin film or one layer of a multilayer coating) deposited by the source on a spinning substrate during a sweep (with fixed velocity) of the spinning substrate across the source. However, until the present invention it had not been proposed to modulate the power applied to a vapor deposition source (or otherwise to modulate the flux distribution of such a source) while sweeping a spinning substrate across the source, in order to deposit on the substrate a layer having a desired thickness distribution profile.
It is expected that the invention will be useful in many applications, including precise deposition of laterally uniform or graded optical interference thin film coatings for EUV lithography, EUV optics, lithography masks, and optical coatings for general applications such as optical communication networks, microscopy, astronomy, and spectroscopy, production of specifically graded coatings on curved optical elements, precise modification of the surface figure of optical elements for fabrication of aspheric optics, and production of extremely uniform films for semiconductor or magnetic recording devices.
In preferred embodiments, the invention is a method and system for determining a source flux modulation recipe for achieving a selected thickness profile of a film to be deposited (e.g., with highly uniform or highly accurate custom graded thickness) over a substrate surface by exposing the substrate to a region containing a vapor of the coating substance (referred to as a vapor deposition xe2x80x9csourcexe2x80x9d of coating material) in which the flux density is controlled. In preferred embodiments, the controlled flux density of the source is determined in accordance with the invention by:
calculating a set of predicted film thickness profiles, each film thickness profile assuming a substrate (spinning at a fixed rate) that sweeps across a source (preferably with a constant sweep velocity but alternatively with time-varying sweep velocity) whose flux distribution is controlled according to different one of a set of source flux modulation recipes (each source flux modulation recipe specifying source flux distribution as a function of time during a time interval in which the substrate sweeps across the source); and
then, determining an optimal (or nearly optimal) source flux modulation recipe to achieve a desired thickness profile (typically by selecting the source flux modulation recipe which corresponds to the predicted film thickness profile which best matches the desired thickness profile).
Typically, each source flux modulation recipe is a power modulation recipe specifying the power applied to the source as a function of time during the time interval in which the substrate sweeps across the source. A thin film having the desired thickness profile can be deposited on a substrate surface by sweeping the substrate across the source while the source flux distribution is controlled in accordance with the optimal source flux modulation recipe.
Preferably, the method includes (and the system is configured to perform) the steps of measuring the flux distribution of the source (resulting from application of each of a number of different power levels to the source) by holding a test piece stationary in a position exposed to the source while a first fixed power level is applied to the source and measuring the deposited layer thickness as a function of position on the test piece, holding another test piece stationary in a position exposed to the source while a second fixed power level is applied to the source and measuring the deposited layer thickness as a function of position on the test piece, repeating the latter step for each additional fixed power level to be applied to the source, and calculating each of the predicted film thickness profiles using each measured source flux distribution (assuming a different source flux modulation recipe for each predicted film thickness profile).
Alternately, the source flux distribution can be measured at only one power level, and other source fluxes can be estimated by assuming a linear dependence of flux with power.
The inventive method for calculating what source flux modulation recipe is needed to give a desired thickness profile on the substrate is applicable not only to flat substrates, but also to both concave and convex curved optics (i.e., optics having nonzero curvature).
Preferably, a computer is programmed to process measured flux distribution data (for a given source or set of sources) to generate a set of predicted coating thickness profiles, each corresponding to a different source flux (or source flux and substrate sweep velocity) function, and to allow the user to conveniently determine an optimal source flux function (or source flux and substrate sweep velocity function) for achieving a predetermined coating thickness profile on each substrate. In general, each source flux function specifies how the flux distribution of the source varies over time (while the substrate sweeps across the source with constant velocity). Each source flux and substrate sweep velocity function specifies the variation over time of both the flux distribution of the source, and the velocity with which the substrate sweeps across the source.
In preferred implementations, the computer is programmed to have a user interface which displays predicted peak-to-valley coating thickness error for each of a number of source flux functions (preferably in the form of a contour map), and a cursor. By manipulating an input device (e.g., a mouse) the user can select any of a number of points on the thickness error display, and in response to each selected point, the computer displays a graph of predicted coating thickness as a function of position on the substrate (for the corresponding source flux function). By inspecting the displayed graph for each of several points on the thickness error display, the user can conveniently determine an optimal source flux function (for achieving a predetermined coating thickness profile on the substrate).
The inventors have recognized that slow spin speeds for the substrate (while the substrate is swept across the source) give rise to a ripple effect (rapid oscillation of coating thickness as a function of radius cross the substrate), which may or may not be acceptable for specific applications.